Method and device for the continuous production of NaAlCl4 or NaFeCl4

ABSTRACT

The invention relates to a method and a device for the production of NADCl 4  in which D is aluminium or iron, where, in a first reaction step, a melt of aluminium or iron is reacted with chlorine gas to give gaseous metal halide, and this is subsequently reacted, in a second reaction step, with solid sodium chloride to give the corresponding compound and is separated off as a melt.

The invention relates to a method and a device for the production ofNaDCl₄, in which D is aluminium or iron, where, in a first reactionstep, a melt of aluminium or iron is reacted with chlorine gas to givegaseous metal halide, and this is subsequently reacted, in a secondreaction step, with solid sodium chloride to give the correspondingcompound and is separated off as a melt

Melts of salts, such as, for example, NaAlCl₄ have various areas ofapplication. Salt melts can be employed as storage medium in heatstorage systems, as heat transfer media, for example in heating baths,for covering and cleaning molten metals, for the electrocoating ofhigh-melting materials or as melt electrolytes in primary batteries, asdescribed in GB 2046506. A further potential application of these saltsis in rechargeable sodium batteries. The salts are employed in batterieswhich have operating temperatures of between 130° C. and 200° C.(Abraham. J. Electrochem. Soc., Vol. 137, 1189-1190, (1990)).

DE 3419279 describes an electrochemical cell in which the cathode matrixis impregnated with a sodium aluminium halide salt melt electrolyte.

A relatively new area of application is the “ZEBRA battery”. Thishigh-temperature cell consists of an electrode of liquid sodium, a betaaluminium electrolyte and an electrode of transition-metal chloride inan NaAlCl4 melt (Cleaver, J. Electrochem. Soc., Vol. 142, 3409-3413,(1995)).

DE 3718920 describes the production of salt melts by the addition of apure metal and an alkali metal halide to the melt The reaction cell isoperated at above the melting point of the salt melt. The alkali metalhalide in the working example is NaCl, the molten alkali metal issodium, and the separator is beta-aluminium oxide. Owing to the use ofpure sodium, special safety precautions, such as working under aprotective-gas atmosphere, have to be taken. The reactions must takeplace in separate cells, since poisoning of the separator by theby-product AlHal₃ formed must be prevented.

All the methods known hitherto for the production of salt melts workbatchwise. A batch procedure has some severe disadvantages compared witha continuous production method. In the case of a batch change, theapparatus has to be opened. The product may then be contaminated by theoxygen from the ambient air, water and dust The batch change results indown times of the plant and thus in a reduced space-time yield. For aneffective discontinuous method, large apparatuses have to be used. Thestart-up process requires correspondingly more energy and time. It hasbeen found that impurities are entrained into the process, in particularduring start-up of plants. FR 2168912 describes a complex purificationmethod for alkali metal aluminium halides. The two-step purificationprocess is composed of an oxygen treatment for degradation of theorganic impurities and an aluminium treatment for precipitation of ironand heavy metals. The aluminium treatment must be carried out under anitrogen or argon atmosphere.

For the production of the alkali metal aluminium halides, the reactionof corresponding aluminium halides and alkali metal halides in a closedtube is described (Friedmann, J. Am. Chem. Soc., 72, 2236-2243, (1950)).A pressure increase to 6-7 atmospheres has been observed in this method,which results in problems (FR 2168912). The apparatuses have to befitted with the appropriate safety precautions.

The object of the invention is to provide a continuous method for theproduction of pure salt melts which excludes disadvantageous ambientinfluences, minimises the energy demand and facilitates an optimumspace-time yield.

A further object is to make large quantities of salt melts available inthe shortest possible time.

The object according to the invention is achieved by a method for theproduction of salt melts, and mixtures thereof, of the general formula

NaDCl₄   (1)

in which

D is Al or Fe,

which, in a first reaction step (i), a melt of aluminium or iron isreacted with chlorine gas to give gaseous metal halide (DCl₃), and thisis subsequently, in a second reaction step (ii), reacted with solidsodium chloride to give the corresponding compound of the formula (I)and is separated off as a melt.

The invention furthermore relates to a device for carrying out themethod, essentially consisting of a reaction vessel (1) containing themelt of the metal D, with a feed device for chlorine gas (2), acollection device for gaseous metal chloride (4) above the reactionvessel (1), and a further reactor vessel (5) which contains sodiumchloride in solid form and is connected to the said collection device.

The products from the method are suitable for use as melt electrolyte inelectrochemical cells, as storage medium in heat storage systems, asheat transfer medium, for example in heating baths, for covering andcleaning molten metals, for the electrocoating of high-melting materialsor as melt electrolytes in rechargeable sodium batteries and primarybatteries.

In the alternative methods, the solids, for example NaCl and AlCl₃, aremixed and warmed to the melting point. The amount of heat necessary forthis purpose has to be supplied from the outside.

Surprisingly, it has been found that the exothermicity of the reactionof aluminium or iron (D) with Cl₂ to give aluminium chloride or ironchloride (DCl₃) can be utilised for the further process for theproduction of NaDCl₄.

In the process according to the invention, DCl₃ (where D=Al or Fe) isformed at temperatures between 700° C. and 1200° C. In contrast toconventional methods, this DCl₃ is fed in gaseous form to an alkalimetal salt bed of NaCl.

It has been found that the accompanying heat of the gas (DCl₃) issufficient to warm the alkali metal salt (NaCl) to the melting point ofthe salt melt NaDCl₄ (I).

An essential advantage of the method is the use of cheaper raw materialsand the utilisation of the heat of reaction being liberated forcontrolling the temperature of the method. Method steps, such as thecondensation of the metal halide (DCl₃), can thereby be saved and theenergy demand for carrying out the process reduced.

It has been found that the continuous performance of the method enablesinterfering environmental influences to be excluded. This enables aconstantly high quality of the product to be established after thestart-up phase.

All continuously operating reaction vessels which appear suitable to theperson skilled in the art can be used for the method. For the reactionwith chlorine gas, a feed device with gas inlet is necessary. Thereaction vessel is provided with a refractory lining. A ceramic liningwhich is insensitive to the materials employed and the high temperaturesis advisable.

The metal D in powder or granule form is provided for the process via asolids metering unit (3).

For collecting the reaction product (DCl₃), a collection device (4),provided with a feed line to the downstream reaction vessel (5), isinstalled above the reaction vessel (1) for the melt

The feed of the reaction product takes place between the upper quarterand the lower quarter of the reactor vessel (5), which contains amixture of metal D and sodium chloride in solid form. Completeconversion of the reactants into the reaction products can thus beensured.

The salt melt formed runs off in a downward direction through the alkalimetal salt bed (NaCl), which is supported by a support grille or acoarse filter plate.

A mixture of solid sodium chloride and metal D in powder or granule formis fed continuously, corresponding to the amount of end product formedand separated off, to the reaction vessel (5) via a solids metering unit(6).

Due to water entrained by the raw materials, undesired HCl gas forms.This can react away to give the metal halide (DCl₃) due to amounts ofthe corresponding metal granules or powder (D) present in the alkalimetal salt bed.

It is advisable for a further reactor vessel (7) with alkali metal saltbed to be installed downstream of the reactor vessel (5) in the flowdirection for purification of the melt in order to allow the metalhalide (DCl₃) formed in turn to react away to give NaDCl₄.

A temperature control device is merely necessary for heating up once inthe start phase and, where appropriate, for cooling. The energynecessary for melting the metal granules (D) is also provided by theheat of reaction.

The method can be carried out continuously or discontinuously asrequired.

A general example of the invention, which is shown in the drawing, isexplained in greater detail below. FIG. 1 shows a reaction vesselcontaining metal melt 1 having a feed device for chlorine gas 2 andsolids metering device 3, collection device for gaseous metal chlorideDCl₃ 4 and reactor vessel 5 containing metal granule or metal powder andalkali metal salt bed, and solids metering device 6, as well as adownstream reactor vessel 7.

Reaction Step i:

For the production of salts conforming to the formula (1) and mixturesthereof, the raw materials can be fed to the reaction vessel (1) inpremixed form via the solids metering device (3). The filling can becarried out under inert gas.

The heatable reaction vessel (1) contains liquid metal melt. Suitablemetals (D) are iron and aluminium. Chlorine gas is fed into the reactionapparatus via the feed device (2). The volume of the melt and the volumeflow of the gas is determined as a function of the requisite residencetime and the desired throughput. A temperature above the melting pointof the metal (D) is set in the reaction vessel (1).

The gaseous metal halide (DCl₃) is fed to the reaction vessel (5) viathe collection device (4) for the reaction product formed.

Reaction Step ii:

The metal halide is fed to the metal granule or metal powder and alkalimetal salt bed between the upper and lower quarter of the reactor vessel(5), preferably between the upper quarter and the centre. A mixture ofmetal granules or powder (D) and alkali metal salt (NaCl) is fedconstantly to the reactor vessel via a solids metering device (6) inaccordance with consumption.

The metal halide (DCl₃) is reacted with the alkali metal salt in thereactor vessel (5) to give NaDCl₄.

Reaction Step iii:

The melt maybe contaminated due to contact with water or atmosphericmoisture. The hydrogen halide formed can react away with the metalgranules (D) added to the salt bed in the reactor vessel (5) to give themetal halide (DCl₃).

Reaction Step iv:

For further processing, the metal halide is passed through the reactorvessel (7). Row takes place from bottom to top through the purificationunit charged with alkali metal salt NaCl. In the process, the metalhalide DCl₃ is reacted with the alkali metal salt NaCl to give thedesired salt NaDCl₄.

The flow from bottom to top through the reactor vessel (7) is notabsolutely necessary. However, it has the advantage that the particlesbecoming smaller due to the reaction are not forced onto the sieve plateby the flow, blocking it. Nevertheless, homogeneous through-flow (plugflow) in the column is ensured in this way. Homogeneous through-flow isan essential prerequisite for complete reaction in the purificationunit.

The example given below is given for better illustration of the presentinvention, but it is not suitable for restricting the invention to thefeatures disclosed herein.

EXAMPLES Example 1 Production of NaAlCl₄

For the production of 1 kg/h of NaAlCl₄, 453.7 g/h of Cl₂ gas are fedfrom a stock vessel to a reaction vessel containing initially introducedaluminium melt At the same time, 172.5 g/h of aluminium granules are fedto the reaction vessel via a solids metering device. The AlCl₃ formingescapes from the reaction vessel in gas form and is fed via a collectiondevice to a reactor vessel which contains a bed of granular common saltand aluminium. This reaction vessel is supplied with 373.8 g/h of NaClby a further solids metering device. A certain amount of granularaluminium may be mixed in with this salt if required for reaction withHCl.

The low-viscosity aluminate formed flows downward out of the reactionvessel and is then advantageously passed through a reactor vessel whichcontains a bed of pure common salt Residues of AlCl₃ react therein,likewise to give the desired product.

I claim:
 1. Method for the production of salt melts, and mixtureshereof, of the general formula NaDCl₄   (I) in which D is Al or Fe,characterized in that, in a first reaction step (i), a melt of aluminiumor iron is reacted with chlorine gas to give gaseous metal halide(DCl₃), and this is subsequently, in a second reaction step (ii),reacted with solid sodium chloride to give the corresponding compound ofthe formula (I) and is separated off as a melt.
 2. Method according toclaim 1, characterized in that the reaction in the second reaction step(ii) is carried out with a mixture of sodium chloride and solid metalgranules or powder (D), where the metal (D) is reacted, in a furtherreaction step (iii), in the melt with any HCl formed to give thecorresponding metal chloride (DCl₃).
 3. Method according to claim 2,characterised in that the chloride formed as by-product in reaction step(iii) is converted into the compound of the formula (I) by reaction withsolid alkali metal chloride in a downstream reaction step (iv) andseparated off as a melt together with the majority of the compound ofthe formula (I) formed after reaction step (ii).
 4. Method according toclaim 1, characterized in that the energy being liberated during thereaction of the metal chlorine gas is utilized for the subsequentreaction with NaCl to give the product.
 5. Method according to claim 1,characterized in that the reaction is carried out continuously. 6.Method according to claim 5, characterised in that the relevant metal Dand the said mixture of metal D and sodium chloride is in each caseadded in solid form continuously corresponding to the amount of endproduct formed and separated off.
 7. Device for carrying out the methodaccording to claim 1, essentially consisting of a reaction vessel (1)containing the melt of the metal D, having a feed device for chlorinegas (2), a collection device for gaseous metal chloride (4) above thereaction vessel (1), and a further reactor vessel (5), which containssodium chloride in solid form and is connected to the said collectiondevice.
 8. Device according to claim 7, characterised in that a solidsmetering unit (3) is provided which contains the metal D in powder orgranule form and is connected to the radon vessel (1).
 9. Deviceaccording to claim 7, characterized in that the gas feed to the reactorvessel (5) takes place between the upper quarter and center of thereactor vessel.
 10. Device according to claim 7, characterized in that asolids metering unit (6) is provided which contains a mixture of solidsodium chloride and metal D in power or granule form and is connected tothe reactor vessel (5).
 11. Device according to claim 7, characterizedin that a further reactor vessel (7), which contains solid sodiumchloride, is installed downstream of the reactor vessel (5).
 12. Use ofthe device according to claim 7 for the production of salt melts of theformula (I) for electrochemical cells, batteries, storage media in heatstorage systems, for covering and cleaning molten metals and for theelectrocoating of materials.